1. Field of the Invention
The present invention relates to a method for forming a polished surface of excellent smoothness on the surface of a vapor-phase synthesized thin diamond film (hereinafter referred to as a "thin diamond film").
2. Description of the Background
It is well-known that an X-ray mask deposited on a semiconductor surface for the preparation of an integrated circuit on the semiconductor can be prepared by:
(a) forming a thin diamond film having an excellent X-ray transmissivity of a thickness of from 1 to 3 .mu.m by the known vapor-phase synthesis technique on the upper surface of an Si wafer (substrate) having a thickness of about 380 .mu.m; PA1 (b) polishing the surface of the thus formed thin diamond film to a prescribed surface roughness; then PA1 (c) forming a membrane composed of the thin diamond film by melting and removing the center portion of the Si wafer from the backside thereof by the use of an etching solution such as a mixture of hydrofluoric acid and nitric acid, thereby forming an Si frame; PA1 (d) sequentially forming an undercoat film, comprising indium-tin oxide, for example, having a high permeability to visible light while preventing the build-up of electric charge caused by charged particles, an X-ray absorber of W-Ti alloy (containing from 1 to 2% Ti), a metallic Cr film serving as an etching mask, and a resist film on the membrane applied by sputtering or spin coating; PA1 (e) forming a pattern of an integrated semiconductor circuit by scanning the resist film with an electron beam; PA1 (f) etching the metallic Cr film by exposure to a mixed gas of chlorine and oxygen, using the thus formed pattern as an etching mask; then PA1 (g) in a state in which the Si frame is cooled to about -50.degree. C., low-temperature etching the X-ray absorber from the bottom thereof, thereby forming an integrated circuit pattern on the semiconductor; and PA1 (h) finally removing the metallic Cr film.
For the purpose of polishing the surface of the thin diamond film in step (b) of the X-ray mask, the method which is generally used comprises the steps of applying a polishing liquid containing powdered diamond particles having an average particle size of about 3 .mu.m dispersed in a solvent comprising a mineral oil such as a light oil or a white kerosene at a ratio within the range of 0.1 to 3 wt. % to a thin diamond film;
applying pressure to the surface of the thin diamond film to bring the same into contact with the surface of a soft stool made of copper or tin;
spraying the polishing liquid onto an exposed portion of the soft stool; and
polishing the stool and/or the thin diamond film while maintaining the powdered diamond particles in a state in which the particles stick to the surface of the stool which contacts the thin diamond film, the polishing occurring by a repeated mutual plane displacement action such as a mutual horizontal rotation.
Recent advances in the field of semiconductor devices are leading increasingly to higher degrees of integration of the devices, and along with this tendency, X-rays which pass through a thin diamond film which composes the membrane of the X-ray mask are required to be free from a shift which would result in a positional shift of an integrated circuit which receives the radiation of this shift. The X-ray shift is largely affected by the smoothness of the polished surface of the thin diamond film. A lower smoothness leads to a larger X-ray shift. Consequently, the thin diamond films must have increasing greater smoothness. With the conventional polishing method as described above, however, the achievable smoothness of a polished surface of the thin diamond film is an Rms (mean square surface roughness) of about 30 nm at most. Polished surfaces having a surface roughness of this order cannot sufficiently cope with the more extensive integration of semiconductor integrated circuits.
In view of these considerations, studies have now been conducted with a view to achieving a further improvement in smoothness of the polished surfaces of thin diamond films, and the following findings have been made. It is possible to achieve further smoothing of the polished surfaces of thin diamond films, as typically represented by an Rms surface roughness within a range of from 0.5 to 10 nm, in contrast to an Rms of only 30 nm in the conventional polishing method, by using, as a polishing liquid, an aqueous solution in which silicon-dioxide (hereinafter referred to as "SiO.sub.2 ") powder particles having an average particle size of from 5 to 1,000 nm are dispersed and distributed in the aqueous solution in an amount of from 5 to 40 wt. %, the aqueous solution having a coefficient of viscosity of from 1 to 200 cP and a pH of from 8 to 12.5; pressing the surface of the thin diamond film and bringing the same into contact with the flat surface of a stool made of an artificial leather such as soft foamed polyurethane, a fabric such as a non-woven felt fabric, artificial or natural soft organic material such as rubber, a synthetic resin or wood, or preferably, soft foamed polyurethane or non-woven felt fabric; and polishing the surface of the thin diamond film by repetitive mutual plane displacement of the stool and/or the thin diamond film.